A wide variety of water-soluble polymers are used to thicken and control rheology of aqueous or water-borne coatings, particularly latex paints. Functions of the water-soluble polymers in water-borne coatings include adding viscosity to the coatings, maintaining the viscosity during storage, and providing desired rheological properties during application of the coatings.
In a typical aqueous coating formulation, 0.1-5.0 wt % of a water-soluble polymer based on the weight of the wet coating is added to the coatings formulation to achieve a desired target viscosity. This target viscosity is typically determined using a Stormer viscometer which reports viscosity in Krebs Unit (KU). For typical water-borne coatings, the Stormer viscosity ranges from 85 to 120 KU. The amount of natural polymers, semi-synthetic polymers, and synthetic water-soluble polymers, known as dry thickener, used to adjust the Stormer viscosity of the coatings to a target viscosity, typically ˜100 KU, is called thickening efficiency (TE) or thickener demand. TE is expressed as weight fraction of the dry thickener with respect to the total weight of the wet coating. North American coatings manufacturers, however, prefer to express TE as pounds of dry thickener required per 100 gallons of wet coatings.
So far as the choice of a thickener is concerned, overall performance and cost of the thickener based on the amount used in the formulation are critical to coatings manufacturers. One way to reduce manufacturing cost by coatings manufacturers is to use low-cost thickeners. Oftentimes, to reduce manufacturing cost, coatings manufacturers use low-cost thickeners even though the use of these low-cost thickeners sacrifices certain desired performance attributes of the coatings. Ultimately the choice of thickener is determined by its unit cost in use subject to fulfilling acceptable performance criteria.
Among the types of water-soluble polymers used in water-borne coatings are natural polymers, semi-synthetic polymers, and synthetic water-soluble polymers. Naturally occurring water-soluble polymers include guar, starch, casein, and alginates. Among semi-synthetic water-soluble polymers, cellulose ethers are the thickeners of choice to formulate water-borne coatings. Examples of cellulose ethers are hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), methylhydroxyethylcellulose (MHEC), methylethylhydroxyethylcellulose (MEHEC), ethylhydroxyethylcellulose (EHEC), carboxymethylcellulose (CMC), hydrophobically modified hydroxyethylcellulose (HMHEC), hydrophobically modified ethylhydroxyethylcellulose (HMEHEC), hydrophobically modified carboxymethylcellulose (HMCMC), hydrophobically modified carboxymethylhydroxylethylcellulose (HMCMHEC), and hydrophobically modified sulfoalkyl hydroxylalkylcelluloses.
The most recent significant development in thickeners is a class of water-soluble polymers having enhanced solution viscosity and thickening efficiency. These water-soluble polymers are called hydrophobically modified water-soluble polymers as they bear a small amount of hydrophobic moieties. In contrast to having inter-chain connections through covalent or ionic linkages, these polymers display inter-chain connections mediated through the congregation of the hydrophobic moieties from different polymer chains. However, these polymers are expensive.
While presently available cellulose ethers produced from purified celluloses, such as chemical cotton (also referred to as purified cotton linters) and wood pulps, can provide the intended performance desired in the marketplace, in many coatings formulations they are not the thickeners of choice because of their high cost in use. To lower coatings manufacturing costs, coating manufactures use less expensive synthetic water-soluble polymers in place of cellulose ethers. These low cost thickeners include polyacrylates and hydrophobically modified polyacrylates. However, water-borne coatings formulated with polyacrylates thickeners tend to form water-sensitive films. For this reason, polyacrylates thickeners are not suitable for thickening water-borne coatings intended for use in coating surfaces that are subject to external insults, such as rain or exposure to high humidity and alkaline materials.
Given the superior performance of cellulose ethers, coating manufacturers would use more cellulose ethers in coatings formulations if these materials could be made more cost effective by either reducing their use levels in the coatings formulations or providing more cost-effective manufacturing through raw material or process innovations.
By designing cellulose ethers with superior viscosity buildup capacity, their use level in water-borne coatings formulations can be reduced. One approach to increase the solution viscosity of cellulose ethers and other water-soluble polymers at a fixed concentration is to cross-link the polymer chains using a suitable cross-linking agent. The nature of the cross-linking can be covalent (permanent bond), electrostatic (ionic bond), hydrogen-bonding or hydrophobic association (pseudo-crosslink) in aqueous solution depending on the nature of the water-soluble polymer and the cross-linking agent. For nonionic water-soluble polymers, cross-linking has to be covalent in nature. However, for ionic water-soluble polymers, the cross-linking can be both covalent and electrostatic provided the polymers possess functional groups capable of reacting with the cross-linker. Since excessive cross-linking tends to form an insoluble species, water-soluble polymers are only modestly cross-linked to maintain the desired solubility. A major problem of using cross-linkers to increase the molecular weight of water-soluble polymers is to control the degree of cross-linking so that no water-insoluble species are formed. Another drawback of using chemically reactive cross-linkers is that due to their high reactivity and toxicity, they often pose significant health risks to those who handle them.
Natural polymer-based thickeners are appealing to manufacturers and consumers in that they are made from renewable resources. For this reason, cellulose ethers continue to be popular. Cellulose is a naturally occurring high molecular weight linear polymer composed of anhydroglucose units that are connected through 1,4-β-glycosidic linkages. Each anhydroglucose unit has three hydroxyl groups that can react with etherifying agents. Cellulose is the structural polymer that provides mechanical properties to all higher-plant cells. In nature, it occurs in the form of microfibrils that are themselves organized into fibers, cell walls, etc. Depending on the source of the natural cellulose, it can be almost pure or it can be admixed with impurities. Cotton lint or staple fiber collected from cottonseed is almost pure cellulose. They are, however, expensive and used almost exclusively in the textile industry. They are not typically used to manufacture cellulose ethers.
Raw cotton linters (“RCL”) are an excellent source of high molecular weight cellulose. Raw cotton linters, also commonly referred to as “linters”, are short fiber residues which are left on the cottonseed after the longer staple (“lint”) fibers are removed by ginning and which have not been subjected to chemical cleaning steps which are typically performed to yield high purity furnish. Linters are shorter, thicker, and more colored fibers than lint. They also adhere more strongly to the cottonseed relative to lint. Raw cotton linters are removed from cottonseeds using a number of technologies including lint saws and abrasive grinding methods, both of which yield suitable materials. The amount of hemicellulose, lignin, colored impurities and foreign matter in the various types of raw cotton linters increases with the number of passes or “cuts” used in removing the inter from the cottonseed. First cut linters typically contain the least amount of impurities and foreign matter and subsequent cuts contain more impurities and foreign matter. Typically, the cellulose content of RCL is about 69-78 wt % as measured by the American Oil Chemists' Society (AOCS) “bB 3-47: Cellulose Yield Pressure-Cook Method”. The balance of noncellulosic materials found in the RCL consists primarily of seed hulls, dirt, field trash, lignin, hemicellulose, wax, fat, protein, moisture and traces of other organic impurities. Some of these noncellulosic materials could result in visible imperfections in any resultant coating if not removed.
Typically, for the manufacture of cellulose ether derivatives, RCL is purified by mechanical and chemical means to yield a high purity furnish. Purified cellulose obtained from RCL is also known as chemical cotton or purified cotton linters. However, processing steps associated the purification of RCL to form chemical cotton or purified cotton linters greatly increases the cost of cellulose ethers made from purified cotton linters. If the RCL is directly converted into its ether derivative, these ether derivatives are typically brown in color, which according to conventional wisdom would be construed as unacceptable for use in making paints and coatings.
For many coatings applications, consistent color and grit-free coatings represent key performance attributes. The typical cellulose content of raw cotton linters is about 69-78 wt %. The rest of the noncellulosic impurities include hemicellulose, lignin, waxes, and inorganic impurities. Currently, noncellulosic impurities, such as hemicellulose and lignin, from RCL are removed by a combination of mechanical and chemical means. However, these treatments occasion changes in fiber morphology and molecular weight loss of cellulose. The fiber morphology, in turn, can alter the mode of reaction of the cellulose with a given etherifying agent leading to different structural features, solubility and solution properties of the resulting ether derivative. In addition, purification of the raw cotton linters to remove noncellulosic impurities poses environmental concerns as the byproducts formed are not innocuous.
Development of novel thickeners based on naturally occurring renewable biopolymers is attractive in view of the future unavailability of petroleum based raw materials that are used to make synthetic water-soluble polymers. The present invention is directed to fulfill this need and provides further related advantages in delivering the rheological properties and other desired properties for various water-borne coatings.